Touch screen panels generally comprise an insulative (e.g., glass) substrate and a resistive layer disposed on the insulative substrate. A pattern of conductive edge electrodes are then formed on the edges of the resistive layer. The conductive electrodes form orthogonal electric fields in the X and Y directions across the resistive layer. Contact of a finger or stylist on the panel then causes the generation of a signal that is representative of the X and Y coordinates of the location of the finger or stylist with respect to the substrate. In this way, the associated touch panel circuitry connected to the panel by a wiring harness can ascertain where a touch occurred on the substrate.
Typically, a computer program generates an option to the user (e.g., “press here for ‘yes’ and press here for ‘no’”) on a monitor underneath a touch screen panel and the conductive electrode pattern assists in detecting which option was chosen when the touch screen panel was touched by the user.
There are typically four insulated individual wires, each extending along and around the edges of the touch screen panel to each corner of the touch screen panel where the insulation is removed and the wire is hand soldered to a terminal electrode on the panel at each corner of the panel. One or more additional layers, usually tape, are often used to secure the wires to the edges of the panel and there may be an insulative layer between the wires and the edge electrodes of the panel to electrically isolate the wires from the edge electrodes.
The problem with such prior art devices are numerous. The soldered joints are often not very reliable and create solder bumps on the smooth surface. Moreover, the act of soldering the ends of each wire to the corner electrodes can damage the electrodes or even crack the glass substrate of the touch panel. Also, this assembly process is labor intensive and costly. Tape may be placed under and/or over the wires. Thus, the assembled touch screen panel does not have a finished appearance. Instead, the taped on wires are readably noticeable and detract from the appearance of the touch screen panel.
For many years, the applicant and others screen printed the edge electrode pattern and the wire traces right on the resistive layer of the touch screen panel.
Normally, the edge electrode pattern is printed directly on the resistive layer of the touch screen panel. Screen printing techniques are used to deposit silver/frit ink directly onto the glass sensor coated with a thin layer of tin oxide. After the pattern is printed, the wet ink is normally dried by heating the panel to about 100° C. for about 5 minutes to reduce the chance that the ink could be removed during subsequent handling. The ink is then fired at about 500° C. for about 20 minutes to sinter and fuse the silver/glass frit mixture to the tin oxide coated glass substrate. This process produces an edge electrode pattern that is mechanically bonded to the touch screen panel and which makes good electrical contact with the tin oxide layer.
The screen printing process, however, can lead to a number of problems when attempts are made to achieve a quality edge electrode pattern. The most severe problem occurs when printing on non-flat or curved glass substrates. In screen printing, a critical parameter in determining the characteristics of the printed pattern is the distance between the printing screen and the substrate. When printing on curved substrates using conventional screen printing equipment, this distance varies with the degree of curvature of the substrate resulting in non-uniform thickness of the printed ink. The whole touch screen cannot be printed at once especially when the area is large or the touch screen has a small radius of curvature. Instead two or more passes and adjustment of the touch screen between passes is required. This problem, in turn, can lead to mismatches in the electrode pattern. Furthermore, standard automated screen-printing equipment, which requires a uniform and repeatable force to push the ink through the printing screen, can not be used with curved screens due to the mismatch of the flat printing screen and the non-flat substrate. In this case, the curved screens can only be printed by manually forcing the ink through the printing screen with a squeegee. The non-reproducible force used in this manual process leads to further variations in the thickness of the printed ink. Frequently, this process also leads to fatal defects in the edge electrode pattern such as complete breaks in the conductive lines of the electrode pattern which subsequently require additional time and labor to rework or reprocess the touch screen panel.
In various fields of technology which are not analogous to the production of touch screens, it is known to apply patterns to a glass surface using a decal transfer method. See, for example, U.S. Pat. Nos. 4,369,063; 4,846,869; 5,346,651; and 2,711,983. These prior art decal transfer methods are typically used only in conjunction with flat substrates. For example, U.S. Pat. No. 4,846,869 describes a method of applying sensors to a windshield by first applying the sensor pattern to the windshield when it is flat and then heating the windshield to shape it into its final curved form. This method can not be used to apply electrode patterns to glass substrates in the manufacture of touch screens because the glass must be curved or bent before the edge electrode pattern can be applied. The reason is that the resistive coating must be applied before the electrode pattern is applied and such a coating cannot withstand the high temperatures which are required to bend glass. U.S. Pat. No. 2,711,983 discloses a method of applying printed electric circuits to curved support surfaces by the use of a decal. In this case, however, the support surface is not a part of the electrical circuit. And it would be undesirable for the decal to make electrical contact with the support surface. In contrast, in the manufacture of touch screens, the support surface, namely the glass substrate, is an integral part of the circuit and it is essential that the electrodes make intimate electrical contact with the resistive tin oxide surface.
After the edge electrode pattern is applied by printing techniques to the touch screen panel, and a protective coating is applied, the next step is to connect individual wires to the corner electrodes at the four corners of the touch screen panel. Typically, the ends of each wire are soldered to the corner electrodes and the individual wires taped to the sides of the panel. In some cases, a noise shield layer, usually in the form of some kind of a tape, is placed in a border configuration around the perimeter of the touch screen panel between the edge electrodes and the individual wires which form a part of the control electronics. In still other cases, a layer of tape is placed on top of the wires again in a border configuration around the perimeter of the touch screen panel.
The result is a touch screen panel with a less than finished appearance since the bulky wires are readily noticeable. Also, the act of soldering the ends of the wires to the corner electrodes can damage the corner electrodes or even damage the substrate. Worse, the solder joints have a tendency to fail.
The applicant then developed a waterslide decal. See U.S. Pat. No. 6,280,552, incorporated herein by this reference. The edge electrode pattern and the wire traces were screen printed on the decal paper which was then coated with a cover coat. The paper was activated with water and removed and the cover coat kept the edge electrode pattern and the wire traces intact as they were transferred to the touch screen. This method resulted in an improvement over prior art screen printing techniques.
The problems associated with this method, however, include the fact that the cover coat must be burned off the panel resulting in an additional manufacturing step and the possibility of damaging the edge electrodes and/or the wire traces. Moreover, the decal tended to float on the glass touchscreen making automation of the transfer procedure difficult. Even by using utmost care, the edge electrode pattern can be distorted when the waterslide decal is used. Also, impurities in the water which contact the touchscreen panel can contaminate the panel. In short, the waterslide decal sometimes resulted in an unpredictable, low reliability, and inaccurate procedure. The above referenced patent also discloses and claims a heat transfer decal. Herein, a new heat transfer decal is disclosed.